Topic #14 . States of Matter

Topic #14. States of Matter

1. Fluids at Rest - Hydrostatics

2. Fluids in Motion - Hydrodynamics

3. Liquids

4. Surface Tension

5. Evaporation and Condensation

6. Solid State

7. Elasticity

8. Thermal Expansion of Matter

9. Plasma

Notes should include:

Fluids at rest - Hydrostatics: A fluid can be defined as any material that flows. A fluid doesn't offer much resistance to changing its shape when it is under pressure. While generally solids are not considered fluids, liquids and solids are. As is done in the study of gas laws, we will look at fluids from an idealistic point of view. Two assumptions will be made to simplify our study. One is that the fluid cannot be compressed and the other is that there is no significant friction among the particles making up the fluid. Fluids exert pressure. Consider the atmosphere. It is exerting pressure on you at all times. We call that atmospheric pressure. When you descend under water you notice an increase in pressure because the water just like the atmosphere exerts an increasing pressure as you descend further down into it. The pressure at a certain depth is exerted in all directions from any point at that depth, even upwards.

In the 17th century, a physician by the name of Blaise Pascal discovered the following principle. Pascal's principle says that the pressure applied to a fluid at any point is transmitted undiminished throughout the fluid. This principle is used in machines that operate using hydraulics. Your mechanic who works on the family car probably has one or more hydraulic jacks or lifts in the repair shop. Most modern vehicles use hydraulic based brake systems. Power is applied to the brakes on the wheels by pressurized fluid activated by you pressing down on the brake pedal. Your power steering system uses a pressurized fluid, also, to amplify your movement of the steering wheel. In general hydraulic systems are used to amplify forces. The equation used to solve such problems is F1 / A1 = F2 / A2 , where F1 represents the force applied to the smaller piston in a hydraulic system and A1 represents the surface area of the piston this initial force is acting on. F2 is the force the fluid is exerting on the larger piston whose surface area is represented by A2. The force F1 ,acting on the first piston, surface area A1 , is both transmitted through the fluid and amplified by the action of the fluid on a larger surface area, A2, resulting in a larger force F2.

Fluids also exhibit a force called buoyancy. Buoyant force is the force a fluid exerts upwards on an object submerged in the fluid. Since gravity pulls objects downwards, even those submerged in liquids, while at the same time the buoyant force is pushing them upwards, objects will appear to weigh less when submerged in fluids. Buoyancy does not cancel gravity, but it does appear to cancel some of the force downwards, because it acts upwards. The equation for buoyant force is described by the statement Fbuoyant = V r g , where V is the volume of an object submerged in the fluid, r is the density of the object, and g is the acceleration due to gravity.

The net weight of an object, that is, its weight submerged in a fluid can be found by the equation Wnet = mg - Fbuoyant.

[An interesting little problem often brought up when studying the affect of fluids on objects is the following one. Imagine a small boat floating in a large swimming pool of water. In the boat are 5 rather large identical bars of gold. If the bars were lowered over the side of the boat into the pool, what would happen to the level of water in the swimming pool?

You can imagine the thinking going on in the minds of people confronted with this question. First of all most everyone realizes that the boat will ride higher in the water displacing less water than it did before, because without the gold in it, it weighs less and doesn't sink down into the water as much. This should cause the level of the swimming pool to go down. On the other hand, the gold bars are now in the swimming pool. They will displace water equal to their own volume and will cause the water in the pool to rise. The question is asking, if after all of this is taken into consideration, does the water level in the pool rise, remain the same, or descend? What do you think? Why?

Consider that in the boat the gold displaced the water by weight (it contributed its weight to the boat), but in the pool it displaces the water by volume and its weight is not an issue. What would could you do to get the gold to float at the surface, if you could use nothing but the gold itself? Wouldn't the gold have to be reshaped so its density was not greater than the water in order for it to float? As it floated it would displace an amount of water equal to its own weight. Is that a greater volume than the actual volume of the gold? If so, how much greater? Would you need to know the density of gold to find the answer? Would that mean that in the problem the water level in the swimming pool would go down as the gold was transferred from the boat to the swimming pool?]

In the third century B.C., a Greek scientist by the name of Archimedes discovered a relationship about volume displaced and buoyancy. His discovery, called Archimedes' principle, says that an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. His principle applies to all objects regardless of their densities. It follows then that a comparison of the density of an object immersed in a fluid and the density of the fluid itself will allow you to determine whether the object will sink or float. For example, if the object's density is greater than the density of the fluid, the object will sink. On the other hand, if the object's density is less than the density of the fluid it will either rise to the surface, if already submerged, or it will ride partially out of the liquid at the surface, just as a boat does. A boat only sinks down into the water until it has displaced a volume of water equivalent to its own weight. Thus, if the sides of the boat are high enough, it will not take on water over the sides and sink. The amount of passengers and cargo a boat (or ship) can carry is a function of how low in the water the boat (ship) can ride in the water without risking swamping the boat (ship). The problem of high waves in a storm is a more complex problem. It usually involves making the ship as water tight as possible above the sides so the waves will just wash over and roll off the decks without entering the ship itself. If that is accomplished the ship will remain afloat even in what is often called high seas.

Fluids in motion - Hydrodynamics: A very interesting but simple activity to demonstrate that moving fluids produce force is to hold a sheet of paper below your lower lip allowing it to hang down vertically. Next, blow air across the top of the piece of paper. You will see the paper lift to a horizontal position and remain there until you run out of breath. What would happen if you didn't run out of breath for a long time? What you are seeing is the result of a difference in pressure between the air above and below the piece of paper. The fast moving air above the piece of paper exerts less pressure on the top side of the piece of paper than the air below the piece of paper. This results in the higher pressure below the paper lifting the paper upwards. Can you think of a situation where this idea is being used to lift something?

Suppose a liquid flows through a pipe of a certain diameter which in turn is connected by a reducer coupling to a pipe with a smaller diameter. If a measured amount of the fluid enters the wide pipe at one end of this assembled pipe, the same amount must come out the narrow pipe at the other end. In such a system the potential energy of the fluid is a function of the pressure the fluid exerts on the walls of the pipe. The kinetic energy of the fluid is a function of the velocity of the fluid as it moves along through the pipe. The total energy in such a system is conserved. This means that if the kinetic energy of the fluid increases the potential energy will decrease. In this pipe assembly as the fluid goes into the narrower portion the fluid's velocity increases (it has to move faster in the narrower portion than in the wider portion) and so does its kinetic energy. As a consequence the fluid's potential energy decreases and as a consequence the pressure of the fluid decreases.

In the 18th century, a swiss scientist by the name of Daniel Bernoulli studied fluids and formulated the following principle. Bernoulli's principle states that when the velocity of a fluid increases, the pressure exerted by the fluid decreases.

By now you may have realized that airplane wings provide a structure to produce lift based on Bernoulli's principle. The blades on the propellers of airplane engines and the blades on the rotors of helicopters also experience a similar type of air flow producing an imbalance of forces that propel the plane forward or lift the helicopter. The design of wings, propeller blades, and the blades on the rotors of helicopters increase the pressure differences above and below the wings or in front of the and behind the blades in order to gain greater lift and thrust. Wings have a curved upper surface and a fairly flat bottom surface. This produces a ratio of pressure on the curved part of a wing (the upper surface) to pressure on flat part of the wing (lower surface) with a value of less one. This imbalance of force produces the lift, an upwards net force. To get the air moving fast enough so the lift is great enough to get the plane in the air, the plane must be propelled forward. The Blade on airplane engines and on helicopter rotors have similar designs. Race cars are also designed to take advantage of this principle. Airfoils often called spoilers, put the curvature on the bottom and the flat part on top, to create a net force downwards so the car's rear wheels hug the road better and the car is less likely to skid. Spoilers are basically upside down wings producing a net force downwards.

Liquids: Liquids differ from gases in several ways. Gases can be fairly easily compressed and their volumes reduced considerably. This is not so with liquids. Liquids are fairly incompressible. Liquids too, have a definite volume, though they will take the shape of the container. Gases have no definite volume, but like liquids take the shape of their containers. The particles in liquids tend to be quite close together, while the particles of gases tend to be quite far apart. The particles in real liquids do exert and experience electromagnetic forces of attraction with one another. These forces are called cohesive forces.

Surface Tension: Surface tension is the tendency of the surface of a liquid to contract. Consider a particle below the surface of a liquid. It is attracted in all directions by other molecules of the same type. Forces are balanced, the net force acting on this particle is essentially zero. Now consider a particle right up at the surface. It has particles pulling on it from the sides and from below it, but no particles are pulling at it from above. As a consequence it experiences a net force having a magnitude greater than zero. There is this net force acting downwards on all of the surface particles. This results in the surface being slightly compressed. These molecules are little like a thin skin over the surface of the liquid. It is a little bit more difficult for an object to penetrate this top layer so light weight objects can be supported by it. For example, an insect called a water strider can walk on the surface of water. Even, small, metal objects, like sewing needles, can be supported by this top layer of water molecules. Surface tension accounts also for the tendency for liquids like water to bead up, that is, form drops instead of spreading out into very thin films when splashed a smooth solid surface. Liquids with weak cohesive forces like a rubbing alcohol tend to have drops that flatten out more than water with its high cohesive forces. Alcohols have low surface tensions and will not so easily support objects like water does.

Adhesion is the attractive force that acts between the particles of different substances. These forces are also electromagnetic attractions. When eater is placed in narrow tubes like graduate cylinders you can see by the shape of the meniscus that the water is attracted to the sidewalls of the cylinder. Also when water is placed in very narrow tubes called capillary tubes, the water appears to move part way up the tube, because of the adhesive forces attracting it up the tube. Capillary action occurs when a narrow glass tube open on both ends is placed in water and the water moves slowly up the tube. This occurs because the adhesive forces are stronger than the cohesive forces. The water stops rising when the weight of the water balances the adhesive forces. Because of the balancing of adhesion by weight, water will rise higher in narrower tubes than wider ones (the weight increases more slowly in the narrow tube as the water rises than in the wider tube). Oil rises up a wick because of capillary action. In a graduate cylinder mercury forms a bulge in the center instead of forming a depression like water, because unlike water the cohesive forces are greater than the adhesive forces.

Evaporation and Condensation: Evaporation is caused by the more energetic particles, those having the highest kinetic energies, overcoming the net cohesive force downwards. They break free of the surface of the liquid. For the overall quantity of liquid present, this means a drop in temperature for the liquid. As the more energetic particles leave, the average kinetic energy of the remaining particles decreases. This means that the temperature has decreased. This is what is meant by the cooling effect of evaporation. This is why skin feels cooler when wet. As the water evaporates the temperature of you skin goes down. A certain, amount of energy is carried away by the escaping molecules. Volatile means the very quick evaporation of liquids that have very weak cohesive forces. Alcohols are such substances that evaporate quickly. These substances tend to cool things down more quickly than water, because they evaporate so much more quickly. Historically, when a patient ran a high fever, doctors would prescribe alcohol rubs, if no other way was available to cool them down. Condensation is the opposite of evaporation. If particles can slow down by losing kinetic energy, they may move slow enough such that when they collide cohesive force make them stick together. Just as evaporation causes a temperature decrease, condensation will cause a temperature increase as each molecule captured contributes its individual kinetic energy to the average kinetic energy of the whole. The pressure of the atmosphere affects evaporation. As the atmospheric pressure is raised, the rate of evaporation decreases. As the atmospheric pressure is lowered, the rate of evaporation increases. If the atmospheric pressure is lowered significantly, water can be made to boil at room temperature.

Solid State: As the temperature of a liquid decreases the particles in a liquid continue to lose kinetic energy the particles slow down to the point where the cohesive forces no longer allow the particles to slide past one another. The particles form a structure called a crystal lattice. The particles do not completely stop but will vibrate in place. Solids do have definite shape and volume. Some materials appear to be solid but do not form a crystal lattice. These materials are classified as viscous liquids. Examples of such substances include glass and paraffin wax. Usually the solid form of a material is denser than the liquid form of the material. Water is an exception to this. Ice is not denser than liquid water, which is why it floats. This is why the ice that forms on ponds lakes and rivers remains at the surface and doesn't sink.

Elasticity: Elasticity is the ability of a solid substance to return to its original shape after forces applied to it distort its shape. Malleability (the ability to be pounded into shape) and ductility (the ability to be pulled into wire) are properties dependent on the elasticity of a material.

Thermal Expansion of Matter: In general material expands when heated and contracts when cooled. Two things happen as a material is heated. One is that the particles move faster, while the second is that they tend to move further apart. As this happens the attractive forces between them becomes weaker, which in turn allows the material to expand. The opposite of this is what happens when things are cooled. As the material cools the particles slow down and move closer together. This allows the attractive forces to increase, which in turn results in the material contracting. Remember, this is true of solids as well as fluids. As the solid material is heated the particles vibrate faster moving greater distances about an average position (center point). Thermal expansion affects many things in the environment.